1. Field of the Invention
The present invention relates to an antenna structure including a capacitive-power-feeding-type radiating electrode and to a wireless communication device including the antenna structure.
2. Description of the Related Art
As a type of antenna provided on a wireless communication device, a surface-mount antenna that is mounted on a circuit board of a wireless communication device and contained and disposed within a case of the wireless communication device exists. In the surface-mount antenna, for example, a radiating electrode that performs an antenna operation is formed on a dielectric substrate.
The frequency characteristics of radio waves of a wireless communication device in which a surface-mount antenna is mounted on a circuit board are not determined only by a radiating electrode of the surface-mount antenna, but are determined by various factors such as a ground electrode or portions of the circuit board having mounted the surface-mount antenna thereon. Thus, the resonant frequency of radio waves for wireless communications carried out by a wireless communication device differs from the resonant frequency of the radiating electrode of the surface-mount antenna. Thus, even when the same surface-mount antenna is mounted, for example, when the type of wireless communication device varies, problematically, the resonant frequency of radio waves for wireless communications carried out by the wireless communication device (hereinafter referred to as the resonant frequency of the antenna) varies.
That is, the peripheral conditions of the surface-mount antenna vary when the type of wireless communication device varies. For example, the size or shape of a ground electrode formed on the circuit board varies, the types of components disposed in the periphery of the surface-mount antenna or the gaps between the surface-mount antenna and peripheral components vary, or the material of the case of the wireless communication device varies. The resonant frequency of the antenna is determined by complex effects of such peripheral conditions of the surface-mount antenna. Thus, when the type of the circuit board having the surface-mount antenna mounted thereon varies so that the peripheral conditions of the surface-mount antenna varies, the resonant frequency of the antenna varies even when the same surface-mount antenna is provided.
As described above, even when the same surface-mount antenna is provided, it is not possible to achieve the same resonant frequency of the antenna when the type of wireless communication device varies. Thus, even when the desired resonant frequency of the antenna is the same, for example, when the type of wireless communication device varies, it is not possible to provide the same surface-mount antenna. Therefore, it is necessary to prepare a custom design of factors such as the size of the radiating electrode of the surface-mount antenna for each type of wireless communication device, which is laborious. According to another method that has been proposed (see, for example, Japanese Unexamined Patent Application Publication No. 10-173426; Japanese Unexamined Patent Application Publication No. 11-312919; and Japanese Unexamined Patent Application Publication No. 2002-335117), the resonant frequency of an antenna is adjusted to a predetermined resonant frequency by preparing custom designs not for a surface-mount antenna but for portions other than the surface-mount antenna, for example, by changing a circuit on a circuit board that is electrically connected to the surface-mount antenna for each type of wireless communication device.
However, according to the method that has been used, in which the resonant frequency of an antenna is adjusted by a circuit on a circuit board, problematically, current loss increases and antenna gain decreases. Furthermore, when a component having a capacitance or an inductance is used for adjusting the resonant frequency of an antenna, if a generic component is used in view of cost, the magnitude of the capacitance or the value of the inductance of the generic component can only be chosen from several predetermined values. That is, in many cases, it is not possible to obtain a capacitor component or an inductor component having an optimal value, so that it has been difficult to adjust the resonant frequency of an antenna precisely to a predetermined resonant frequency.